It is known to use comb polymers in hydraulic cement compositions. For example, in U.S. Pat. No. 5,393,343, incorporated fully herein by reference, Darwin et al. disclosed an EO/PO type comb polymer useful as a superplasticizer for retaining in concrete a high degree of slump (e.g., high flowability) over a sustained period of time without imparting significant set retardation. As used herein, the term "EO/PO" is synonomous with the term "oxyalkylene group" and serves as a convenient short-hand to designate polyoxyalkylene groups (e.g., ethylene oxide/propylene oxide copolymers). Thus, for present purposes, the term "EO/PO type comb polymer" means and refers to a polymer having a carbon backbone to which are attached pendant carboxylate groups (which function as cement anchoring groups in the cementitious mixture) and also comparatively longer pendant groups or "combs" such as ethylene oxide (EO) groups, propylene oxide (PO) groups, and/or a combination of EO/PO groups. The pendant groups may be ionic or non-ionic.
U.S. Pat. No. 5,393,343 also disclosed that a variety of conventional cement additives, such as "antifoaming agents," may be mixed with the cement composition prior to, along with, or subsequent to the addition of the superplasticizer. It is know that EO/PO type comb polymers commonly entrain excessive air in the cementitious mixture, and thus antifoaming agents were needed. Antifoaming agents exhibit very limited stability in aqueous solutions because they are hydrophobic and tend to separate in the liquid admixture solution which contains the comb polymer. Commonly used antifoaming agents (or "defoamers") include EO/PO type defoamers (especially ones rich in PO units), silicones, tri-butyl phosphate, and alkylphthalates. The phase instability of the comb polymer and antifoaming agent combination requires that they be placed into separate tanks and mixed just before incorporation into the cement composition (e.g., concrete), or, alternatively, that they be contained in a tank requiring constant stirring. In any event, the admixture solution has a short shelf life, creating inconvenience and added expense for the concrete preparation operation.
European Patent Application No. 0 725 043 (A2) of Sandoz Ltd. teaches that antifoaming agents, such as certain polyalkylene glycols or derivatives thereof, can be grafted as side chains directly onto a carbon backbone along with side chains comprising an oligoalkyleneglycol and/or polyalcohol to form the comb polymer. This approach relies on reproducible hydrolysis of the ester linkage to allow predictable release of the defoaming agent from the carbon-containing backbone.
UK Patent 2 280 180 A owned by Sandoz Ltd. disclosed the use of a polymer, which comprised units derived from unsaturated carboxylic acid monomers, in an aqueous solution wherein an oxyalkylene-based defoaming agent present during the polymerization of the monomers becomes dissolved or dispersed in particles of no greater than 20 .mu.M diameter. Such a dispersion, however, may be phase unstable at elevated temperatures and during prolonged storage. Commonly known concrete defoamers, such as tri-butyl phosphate, even when dispersed are known to coalesce and phase-separate.
In view of the foregoing disadvantages of the prior art, a novel concrete admixture comprising an EO/PO type comb polymer and antifoaming agent, which allows for predictable air control, and method for making the same, are needed.